You are correct. Generally, Larian does the German and English versions of their games, and the publishers in other regions are responsible for localization (in some cases a full translation, or just the text with subtitles for English audio). The German version of D2:ED included the English text, though without official support for switching languages.
In Beyond Divinity some people have had partial success converting to English by replacing some of the language related files and then making a registry edit to install the English patch. Only a couple people have tried this with DD, so I'm not sure how well it will work. Differences with unicode cause problems with the Thai version, but the German version should do better.
If you want more details on the Thai translation attempt, see the section of the following old topic, starting with Dragonfire's post;
Technical problems, help us to help youThe forum server/domain was changed awhile ago; any old links to "lariancom.siteprotect.net" just have to have that part of the URL changed to "larian.com" to work properly. The troubleshooting post mention in the above topic has been updated, and the working link is
Troubleshooting Tips (& manual install procedure).